The invention relates to a photopolymer formulation comprising chemically crosslinked matrix polymers, writing monomers and a photoinitiator system, to the use of the photopolymer formulation for producing holographic media and also to a process for producing exposed holographic media.
WO 2008/125229 A1 describes photopolymer formulations of the type mentioned at the beginning. They comprise chemically crosslinked polyurethane-based matrix polymers, acrylic-based writing monomers, and also a photoinitiator system.
Media formed from the known photopolymer formulation can be exposed to electromagnetic radiation to form holograms therein. However, the application primarily describes formulations for data storage applications wherein the holograms are produced using laser light in the ultraviolet region. The holograms obtained are difficult or impossible to see with the naked eye. It does not describe formulations yielding bright holograms which are readily and directly perceivable by the human eye.
To produce bright and easily visible holograms, it is generally desirable to seek a high refractive index difference Δn and a low thickness d for the medium while maximizing the diffraction efficiency DE. That is, increasing Δn increases the latitude to engineer the layer thickness without incurring a loss of DE. Therefore, the optimization of Δn in the production of photopolymer formulations for bright, visible holograms is of outstanding importance (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996).
It is an object of the present invention to provide a photopolymer formulation into which bright holograms quickly and simply perceivable by the unaided human eye can be exposed.